For detecting an opening ratio of a throttle valve by a throttle sensor, it is generally necessary to accurately determine an output from a throttle valve sensor relative to a reference output for a fully closed position of the throttle valve. One such a throttle valve opening detecting device is disclosed in, for example, Japanese Patent Unexamined Publication No. 56-107,926 entitled "DEVICE FOR DETECTING ENTIRE CLOSING OF THROTTLE VALVE OF INTERNAL COMBUSTION ENGINE" laid open Aug. 27, 1981. The throttle valve opening detecting device taught by the above mentioned Japanese Patent Unexamined Publication detects the latest minimum value of a varying output voltage from a throttle valve opening sensor which is taken for a reference output for a closed position of the throttle valve.
Practically, however, such throttle valve sensors sometimes detect output voltages lower than an output voltage to be provided when the throttle valve is actually fully closed. This fluctuation is due to a drop of power of a battery, noises or cranking. If such a fluctuated or incorrect output voltage is taken for a reference output voltage for the fully closed throttle valve for controlling an automatic transmission so as to change gear ratio or to lock or unlock a lockup torque converter, a shift point at which the automatic transmission upshifts into high or downshift into low range undesirably varies or it is temporarily disabled to timely release a lockup condition of the automatic transmission, in particular a lockup torque converter in association with the automatic transmission.
Also an idle switch is known which is actuated when a throttle valve is fully closed to set an output from a throttle valve opening sensor as a reference output for a fully closed position of the throttle valve. However, the provision of such idle switches needs wire for itself, resulting in a loss of space and cost.